netcat很强大

Netcat or nc is a networking utility for debugging and investigating the network.

This utility can be used for creating TCP/UDP connections and investigating them. The biggest use of this utility is in the scripts where we need to deal with TCP/UDP sockets.

In this article we will learn about the netcat command by some practical examples.

1. Netcat in a Server-Client Architecture

The netcat utility can be run in the server mode on a specified port listening for incoming connections.

$ nc -l 2389

Also, it can be used in client mode trying to connect on the port(2389) just opened

$ nc localhost 2389

Now, if we write some text at the client side, it reaches the server side. Here is the proof :

$ nc localhost 2389
HI, server

On the terminal where server is running :

$ nc -l 2389
HI, server

So we see that netcat utility can be used in the client server socket communication.

2. Use Netcat to Transfer Files

The netcat utility can also be used to transfer files. At the client side, suppose we have a file named ‘testfile’ containing :

$ cat testfile
hello test

and at the server side we have an empty file ‘test’

Now, we run the server as :

$ nc -l 2389 > test

and run the client as :

cat testfile | nc localhost 2389

Now, when we see the ‘test’ file at the server end, we see :

$ cat test
hello test

So we see that the file data was transfered from client to server.

3. Netcat Supports Timeouts

There are cases when we do not want a connection to remain open forever. In that case, through ‘-w’ switch we can specify the timeout in a connection. So after the seconds specified along with -w flag, the connection between the client and server is terminated.

Server :

nc -l 2389

Client :

$ nc -w 10 localhost 2389

The connection above would be terminated after 10 seconds.

NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no effect and hence the connection remains open forever.

4. Netcat Supports IPV6 Connectivity

The flag -4 or -6 specifies that netcat utility should use which type of addresses. -4 forces nc to use IPV4 address while -6 forces nc to use IPV6 address.

Server :

$ nc -4 -l 2389

Client :

$ nc -4 localhost 2389

Now, if we run the netstat command, we see :

$ netstat | grep 2389
tcp        0      0 localhost:2389          localhost:50851         ESTABLISHED
tcp        0      0 localhost:50851         localhost:2389          ESTABLISHED

The first field in the above output would contain a postfix ’6′ in case the IPV6 addresses are being used. Since in this case it is not, so a connection between server and client is established using IPV4 addresses.

Now, If we force nc to use IPV6 addresses

Server :

$ nc -6 -l 2389

Client :

$ nc -6 localhost 2389

Now, if we run the netstat command, we see :

$ netstat | grep 2389
tcp6       0      0 localhost:2389          localhost:33234         ESTABLISHED
tcp6       0      0 localhost:33234         localhost:2389          ESTABLISHED

So now a postfix ’6′ with ‘tcp’ shows that nc is now using IPV6 addresses.

5. Disable Reading from STDIN in Netcat

This functionality can be achieved by using the flag -d. In the following example, we used this flag at the client side.

Server :

$ nc -l 2389

Client :

$ nc -d localhost 2389
Hi

The text ‘Hi’ will not be sent to the server end as using -d option the read from stdin has been disabled.

6. Force Netcat Server to Stay Up

If the netcat client is connected to the server and then after sometime the client is disconnected then normally netcat server also terminates.

Server :

$ nc -l 2389

Client :

$ nc localhost 2389

Server :

$ nc -l 2389
$

So, in the above example we see that as soon as the client got disconnected the server was also terminated.

This behavior can be controlled by using the -k flag at the server side to force the server to stay up even after the client has disconnected.

Server :

$ nc -k -l 2389

Client :

$ nc localhost 2389
^C

Server :

$ nc -k -l 2389

So we see that by using the -k option the server remains up even if the client got disconnected.

7. Configure Netcat Client to Stay Up after EOF

Netcat client can be configured to stay up after EOF is received. In a normal scenario, if the nc client receives an EOF character then it terminates immediately but this behavior can also be controlled if the -q flag is used. This flag expects a number which depicts number of seconds to wait before client terminates (after receiving EOF)

Client should be started like :

nc  -q 5  localhost 2389

Now if the client ever receives an EOF then it will wait for 5 seconds before terminating.

8. Use Netcat with UDP Protocol

By default all the sockets that nc utility creates are TCP protocols but this utility also works with UDP protocol. To enable UDP protocol the -u flag is used.

Server :

$ nc -4 -u -l 2389

Client :

$ nc -4 -u localhost 2389

Now, both the server and client are configured to use UDP protocol. This can be confirmed by the following netstat command. So we see that this connection is now using the UDP protocol.

$ netstat | grep 2389
udp        0      0 localhost:42634         localhost:2389          ESTABLISHED
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